Abstract

It is now sixteen years since Macrae and Moorehouse1 first described an NAD-dependent enzyme in plant mitochondria which oxidatively decarboxylated malate to pyruvate. Since then it has been established that plant mitochondria normally possess two malate oxidising enzymes; NAD-dependent malate dehydrogenase (EC 1.1.1.37), and NAD-dependent malic enzyme (EC 1.1.1.39). Both enzymes are located in the mitochondrial matrix2. Therefore, they may compete for their substrates, and for access to the electron transport chain, in a similar fashion to that described for glycine with TCA* cycle substrates in which glycine is preferentially oxidised3. A knowledge of the extent to which these malate oxidising enzymes compete with each other, and how their activity is regulated, is important because malate dehydrogenase produces oxaloacetate which is an essential substrate for the TCA cycle. Therefore, if the activity of these enzymes were unbalanced, the TCA cycle might cease to operate. Furthermore, malic enzyme plays an important role in the decarboxylation of malate in some CAM and C4 plants.

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